Ring-opening reactions cyclobutane

It has been found that certain 2 + 2 cycloadditions that do not occur thermally can be made to take place without photochemical initiation by the use of certain catalysts, usually transition metal compounds. Among the catalysts used are Lewis acids and phosphine-nickel complexes.Certain of the reverse cyclobutane ring openings can also be catalytically induced (18-38). The role of the catalyst is not certain and may be different in each case. One possibility is that the presence of the catalyst causes a forbidden reaction to become allowed, through coordination of the catalyst to the n or s bonds of the substrate. In such a case, the... 

A cyclobutane ring-opening of the photoadduct (426) by the reaction of BF3 Etfi in refluxing benzene gave isopropenylcyclohexenone (427). The reaction could be applied to more complex compounds. Thus, (428), (430), (432) and (434) were converted accordingly to (429), (431), (433) as well as (435) 145). 

If hydrogen occupies all sites, the dual-site mechanism may operate over two adjacent /2g sites 42). The importance of active site periodicity and the screening of the adjacent C—C bond is valid in this case, too. This (assumedly adsorbed) hydrogen does not participate in C5 cyclic reactions. There is some indication, however, that it might be mobilized for cyclobutane ring opening 97, 97a). 

Using all species in their liquid phase this reaction is found to be exothermic by —7 kJmol while the related ring-opening reaction of cyclobutane is exothermic by ca —106 kJmoU. ... 

Thus, the ring-opening reaction of cyclobutyl derivatives is usually not used for preparative purposes. It is, on the contrary, an undesirable back reaction in the synthesis of cyclobutanes by homoallyl rearrangement (see Section 7.A.2.I.).3... 

Cyclobutane formation, occasionally in fairly low yield due to ring-opening reactions, also occurred by photochemical [2 - - 2] cycloaddition of vinylcyclopropane derivatives to an a, -unsaturated ketone moiety, " a cyclobuta-1,3-diene, a 1,4-benzoquinone, 2-acylthiophenes, and on irradiation of e t/o,enr/o-2,4-bis[( )-2-(methoxycarbonyl)vinyl]bi-cyclo[1.1.0]butane. A formal [2-1-2] cycloaddition also occurred on reaction of tricyclo[3.1.0.0 ]hex-3-ene with methyl 6-oxo-5-phenyl-l,3,4-oxadiazine-2-carboxylate to give methyl 9-oxo-10-phenyl-8-oxapentacyclo[4.4.0.0 ". 0 .0 °]decane-7-carboxylate, albeit in very low yield, after nitrogen extrusion. ... 

However, the apphcability of this cycloaddition is rather limited because of the side reactions such as ene reactions, conjugate addition reactions, and ring opening reactions of the produced cyclobutane derivatives. There was no general Lewis acid-catalyzed [2+2] cycloaddition reaction, until Takeda [25] and Narasaka et al. [26,27,28,29,30,31,32,33] found that alkenyl sulfides react with a wide variety of electron-deficient olefins in the presence of a Lewis acid. 

Cyclobutanes are also known [129-131] to undergo similar types of ring opening reaction, and several examples on this topic along with their applications may be found in the comprehensive review article of Mattay [6]. 

In four-membered heterocycles, the ring strain is less than in the corresponding three-membered compounds and is approximately equal to that found in cyclobutane. Nevertheless, ring-opening reactions forming acyclic products predominate. At the same time, analogy with the reactivity of the corresponding aliphatic compounds (ethers, thioethers, secondary and tertiary amines, imines) becomes more evident. 

Thus, the unusual ring-opening reactions of cyclopropane and cyclobutane (relative to other alkanes and cycloalkanes) are thermodynamically reasonable. 

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